A known specimen holder for a transmission electron microscope is constructed as shown in FIGS. 7(a) and 7(b). In FIG. 7(a), the specimen holder has an insertion rod 41. A specimen-holding portion 42 is formed at the front end of the rod 41 and provided with a hole 43 permitting passage of the electron beam. A piece of mesh 44 on which a specimen S is placed is held within the hole 43 by a hold-down member 45 such as a spring or screw. The holder is held to a goniometer (not shown). The front end of the holder is inserted into the location of the optical axis O of the electron beam between the magnetic pole pieces 46 by the goniometer. In this location, the front end is irradiated with the electron beam. The goniometer is capable of moving the specimen within a plane perpendicular to the optical axis and tilting the specimen from the optical axis.
In the above-described side-entry specimen holder, the specimen-holding portion 42 may be provided with a plurality of holes permitting passage of the electron beam as shown in FIG. 7(b) to observe and compare plural specimens. However, the number of the holes is few at the most. Where more specimens are observed and compared, the specimens already observed are withdrawn from the holder, and then they are replaced with a new set of specimens. Alternatively, plural specimen holders are prepared to replace specimens.
Where the specimens are replaced after they are once detached from the holder, the detached specimens must be replaced under the same conditions as the conditions established prior to the detachment. For example, a region of a specimen which is observed at a magnification of tens of thousands must be placed at the previous azimuth angle, or at the previous angular position. This operation is impossible to achieve manually. Also, in the case of a very thin specimen, it might be installed upside down. In this way, it is difficult to reproduce the observational conditions faithfully.
Where a plurality of specimen holders are prepared, the cost of the instrument is increased. Hence, this scheme is quite uneconomical.
Furthermore, the replacement operation is extremely cumbersome to carry out, because specimens observed by transmission electron microscopy are very thin, generally less than 1 .mu.m, for example, and because the diameter of the opening spaces in the mesh is only 3 mm. Therefore, it takes a long time for an unskilled operator to replace the specimens. In addition, there is a possibility that he or she destroys valued specimens during replacement operation.